1. Field of the Invention
This invention relates to fasteners and, in particular, to tubular fasteners and methods of attachment to sheet metal panels and certain copolymer and plastic panels.
2. Description of the Prior Art
In the manufacture of automotive products it is often necessary to join component parts, one of which is a panel, to one another by the use of threaded fasteners. Bolts or screws are commonly threaded into suitable nuts secured to the panel. The prior art in methods of attachment encompass various types of fasteners including nuts, clinch nuts and pierce clinch nuts. They are attached by welding or by a variety of mechanically clinched, extruded, or swaged techniques whereby the nut body or a portion thereof, cooperating with a die member, effects an attachment.
Clinch nuts are most commonly attached to sheet material by one of two methods. In the first method the nuts are punched through the sheet material, clinching the nut around the periphery of the hole punched out by the nut. Using this method of attachment reduces the load bearing surface area of the nut abutting against the sheet material. When power tools are used to insert a bolt or screw into the attached nut, there is a tendency to dislodge the nut from the panel because of the low push-off strength of these fasteners.
In the second method, the nuts are clinched onto the surface of the sheet material while simultaneously having a punch in the bore of the nut punch out a slug in the sheet material. This method of attachment offers an increased area of load bearing surface abutting against the sheet material. It also necessitates the use of a thread cutting bolt with the fastener, because the diameter of the punch in the bore of the nut cannot be larger than the minor diameter of the threaded bore. The use of thread cutting bolts is shown by the U.S. Pat. Nos. to Pouch et al, U.S. Pat. Nos. 3,829,957 and 3,926,236, and Russian Pat. No. 506,317. The use of thread-cutting bolts, however, reduces the performance of the nut because of the relatively low push-off strength it exhibits. Also, the use of such bolts is more costly and it is more difficult to control tolerances of the fastener.
In either method, the nuts are relatively heavy, making their use undesirable in automotive assembly where there has been an effort to reduce weight to increase fuel economy. With the quest for lighter weight vehicles, thinner sheet panels are being used. Inadequate pull-through strength has been cited as a weakness of clinch and pierce and clinch type fasteners.
The prior art patents of Schleicher, U.S. Pat. No. 3,900,937; Marquis, U.S. Pat. No. 4,059,897; Williams, U.S. Pat. No. 2,254,558; Double, U.S. Pat. No. 3,299,500; Double et al, U.S. Pat. No. 3,315,345; Grube, U.S. Pat. No. 3,704,507; and Steward, U.S. Pat. No. 3,878,598 all teach methods of attachment wherein first a panel of material is severed by punching, shearing, or lancing and then a nut body is deformed. However, any fastener attachment which causes the panel material to be severed increases the potential for seepage of corrosive liquids through the severed panel.
Barrel-type, clip-on fasteners have been used as a lightweight, low-cost alternative to clinch nuts. The present clip-on method for mounting these fasteners, however, restricts their application to holes near the edge of the sheet material. Furthermore, the clip-on mounting method does not provide as secure a mount as the clinch nuts.